Non-chromate corrosion inhibitors for aluminum alloys

ABSTRACT

A non-chromate, corrosion-inhibiting coating composition for metal surfaces includes at least one inhibitor selected from the group consisting of phosphates, phosphosilicates, silicates, and mixtures thereof, at least one inhibitor selected from the group consisting of titanates, zinc salts, and mixtures thereof, and a carrier for these inhibitors, the carrier capable of placing the inhibitors in proximity with the metal surface. In a preferred embodiment, the coating composition further includes a borate such as boric acid, and a succinate. A preferred phosphate includes calcium dihydrogen phosphate, and sodium titanium oxide is a preferred titanate. The zinc salt can include zinc phosphate or zinc cyanamide.

This application claims the benefit of U.S. Provisional Application Ser.Nos. 60/005,027 and 60/005,028 filed Oct. 10, 1995.

FIELD OF THE INVENTION

This invention relates generally to methods and compositions forinhibiting corrosion, and in particular, to methods and compositions forinhibiting corrosion of metals, especially aluminum and other lightmetal alloys used in aircraft. Most specifically, the present inventionrelates to a non-chromate containing, corrosion-inhibiting coatingcomposition capable of protecting a wide variety of metal surfaces.

BACKGROUND OF THE INVENTION

The U.S. Environmental Protection Agency has identified the metalfinishing industry as one of the one most significant contributors toenvironmental pollution in the United States and, in all likelihood,throughout the rest of the world. This is because the materialscurrently most used in metal finishing include chromium, cadmium, zinc,lead, copper, nickel, chromates, and many toxic or polluting volatileorganics.

The aircraft industry, being one of the largest of the industrialfinishers, provides an example of the environmental impact of theseprocesses. In a 1990 report, Tinker Air Force Base (Oklahoma) reportedproducing and treating 1.4 million gallons of industrial waste water perday mostly from metal finishing. The principal contaminants werechromium, nickel, copper, cadmium, lead, zinc, tartrates, EDTA,phosphate, and ammonia. It is estimated that the cost of disposing ofthese wastes is approximately $220 per ton, which equates to severalthousand dollars per day for this one site.

The chromate ion, which is an excellent corrosion inhibitor, has beenone of the most widely used for almost a hundred years. It is generallyused as a pigment in corrosion inhibitive paints, wash primers, sealantsand caulks. It is also used in chromate conversion coatings, etchingsolutions, and in sealing anodized and phosphate coatings.

For the past ten years, however, chromate has been recognized as toxicand carcinogenic, and because of its health risks, has become highlyregulated. With pressure for elimination being exerted by governmentregulations, continued use of chromate will incur ever increasingeconomic penalties. Hence, there is urgent need for non-toxicsubstitutes, both from economic and environmental standpoints.

Currently, the most widely used inhibitors for passivating aerospacealuminum alloys and other light metals are the alkaline earth and zincsalts of hexavalent chromium. They vary mostly in their degree of watersolubility (in the order Mg>Ca>Sr≧Zn) and to a much smaller extent intheir pH (Sr≧Ca>Mg). The chromate anion is the active species, reliablyperforming four necessary functions to be more fully described below. Inaddition, all of the above-described hexavalent chromium salts enhanceadhesion in many paint and sealing systems.

Chromate performs four functions, thereby making it a desirableinhibitor. These functions include:

1. Rapid exit from a carrier matrix, such as paint or sealant.

2. Adsorption of the chromate anion on the bare metal or metal oxide.This alters space charge distributions at the interface, lowering theisoelectric point of the protective anodic metal oxide layer naturallyforming on active metals. This repels chloride attack, and/or shifts thecorrosion potential of that metal and/or its pitting potential in thenoble direction.

3. Instead of oxygen reduction at cathode sites, reduction of chromium(VI) anion occurs to form an acid-insoluble ("persisting"), chromium(III) oxide layer at cathode sites. This fills oxide voids over cathodesites and blocks further corrosion reaction.

4. Buffering the pH or neutralizing increasing acidity at themetal/electrolyte interface which comes from metal oxidization in theabsence of air. Increasing acidity accelerates corrosion exponentially.

In addition to these desirable inhibitive functions, chromate salts havethe advantages of: (1) promoting adhesion at the metal/resin interfaceunder a coating or sealant compound; (2) working well on a wide varietyof metal and alloy substrates because they passivate both anodically andcathodically; (3) being relatively neutral in pH; and (4) being strongoxidizers only in acid conditions, and thus not destroying or stronglyreacting with the resin matrix in which they are placed.

The prior art discloses a number of non-chromate species which have someinhibitive capabilities. For example, U.S. Pat. No. 5,126,074 discloses"hydrogen phosphate" anions as exhibiting corrosion inhibitive activityon aluminum. The patent further discloses the use in coatings ofalkaline earth monohydrogen phosphates, together with a carbonate of thesame alkaline earth metal and an additive of alkaline fluorosilicate orfluoroborate or alkali or alkaline fluoride. This combination is said toprevent filiform corrosion on aerospace aluminum alloys.

Other references cite the dihydrogen phosphate anion as adsorbing onalumina and lowering its isoelectric point (IEP) from pH=9 to pH=5.Lowering the IEP of aluminum oxide on aluminum metal has been shown toincrease its resistance to pitting. Since this species also exhibitsbuffering capability, it performs functions 2 and 4.

U.S. Pat. No. 2,624,708 discloses carcinogenic mercaptobenzothiazole(MBT) as an inhibitor for aluminum and steel. Sulphur and mercaptogroups which are "soft bases" are known to have a high affinity fornoble and other bare metal ("soft acid") surfaces. They are effectiveinhibitor structures under acidic conditions where no oxide is present.By itself, this species performs only function 2.

U.S. Pat. Nos. 4,457,790 and 5,125,989 disclose the use of Mannichadducts of vegetable tannin or polyalkenyl phenols to "conversion coat"aluminum. A titanium ion or compound such as fluorotitanic acid, amongothers, is claimed as a co-reactant. U.S. Pat. No. 5,129,967 disclosesminute catalytic amounts of dihydrohexafluorotitanic acid andhydrofluoric acid used with much larger amounts ofdihydrohexafluorozirconic acid and polyacrylic acid. These patents referto usages on aluminum and/or aluminum alloys.

U.S. Pat. No. 5,314,532 discloses zinc, cobalt, nickel and leadcyanamide pigments as exhibiting corrosion inhibitive effects on silverand thin mirror coatings. Bare, oxide-free copper would be expected toshow adsorption characteristics somewhat analogous to silver, especiallyin an acidic, crevice environment. As with silver groups, theavailability of electrons on the cyano group act as a "soft base" onbare metal "soft acid" surfaces, performing function 2.

The present invention provides a corrosion-inhibiting coatingcomposition which performs many, if not all of the same functions as achromate-containing composition, but without the need for the harmfulchromate species. The problem solved by this invention is theelimination of toxic hexavalent chromium salts which are known to behuman carcinogens, as corrosion inhibitors from treatment solutions,coatings, and sealants used on aluminum and other metal alloys. Thepresent invention provides for the synergistic combinations ofnon-chromate inhibitors for aerospace aluminum alloys and other metalsurfaces which can be incorporated into both curable and non-curingsealants and into curable primer and unicoat systems. This synergisticcombination of inhibitors can also be incorporated into water-containingor water-absorbing fluids that might cause corrosion when used in theproximity of metal such as de-icing liquids and coolants. These andother advantages of the present invention will be readily apparent fromthe description, discussion and examples which follow.

SUMMARY OF THE INVENTION

There is disclosed herein a non-chromate, corrosion-inhibiting coatingcomposition for metal surfaces. As used herein, "coating" is used tomean any composition which can cover a substrate, or which can place theinhibitors in proximity with a substrate. The composition comprises atleast one inhibitor selected from the group consisting of phosphates,phosphosilicates, silicates, and mixtures thereof, at least oneinhibitor selected from the group consisting of titanates, zinc salts,and mixtures thereof, and a carrier for these inhibitors, the carrierbeing capable of placing the inhibitors in proximity with the metalsurface.

In particular embodiments, the coating composition of the presentinvention can further comprise a borate, such as boric acid, and/or asulfur-containing succinate such as (2-benzothiazolylthio)succinic acidor amine salts thereof. A preferred phosphate is calcium dihydrogenphosphate. A preferred phosphosilicate is calcium, strontium zincphosphosilicate. Sodium titanium oxide is a preferred titanate. Zincphosphate and/or zinc cyanamide are the preferred zinc salts. Thecarrier comprises a solution or polymer matrix which adheres well tometal substrates and is capable of placing the inhibitors in closeproximity with the metal surface. The coating composition of the presentinvention may also include ancillary ingredients such as pigments,rheological agents, and other performance additives.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the synergistic combinations of twoto six individual corrosion inhibitors contributing two to five separatefunctions to the inhibition of corrosion on metals. The coatingcomposition is particularly well suited for protecting light metalalloys, such as the aluminum alloys used in aircraft.

In the broadest sense, the present invention comprises at least oneinhibitor selected from the group consisting of phosphates,phosphosilicates, silicates, and mixtures thereof, at least oneinhibitor selected from the group consisting of titanates, zinc salts,and mixtures thereof, and a carrier for the inhibitors, the carriercapable of placing the inhibitors in proximity with a metal surface. Thecoating composition of the present invention can further comprise aborate, such as boric acid, and/or a succinate, preferably asulfur-containing succinate. The succinate comprises a compound selectedfrom the group consisting of (2-benzothiazolylthio)succinic acid, thefatty amine salt of (2-benzothiazolylthio)succinic acid, and mixturesthereof. In a preferred embodiment, the inhibitors are loaded into thecarrier to achieve 3-40% by volume in a dried film, more preferably5-25% by volume in a dried film, and most preferably 10-20% by volume ina dried film.

In a preferred embodiment, the phosphate comprises a dihydrogenphosphate, most preferably a compound selected from the group consistingof calcium dihydrogen phosphate, potassium dihydrogen phosphate,ammonium dihydrogen phosphate, sodium dihydrogen phosphate and mixturesthereof. The phosphate can also comprise a pyrophosphate, preferablysodium pyrophosphate, or a monohydrogen phosphate, preferablydipotassium monohydrogen phosphate.

The phosphosilicate of the present invention preferably comprises acompound selected from the group consisting of calcium, strontiumphosphosilicate, calcium, strontium, zinc phosphosilicate, and mixturesthereof. The silicate of the present invention preferably comprises anorthosilicate, most preferably tetrasodium orthosilicate.

In a preferred embodiment of the present invention, the titanatecomprises a titanium oxide, more preferably sodium titanium oxide. Thezinc salt preferably comprises a compound selected from the groupconsisting of zinc phosphate, zinc cyanamide, and mixtures thereof.

When used in the coating composition of the present invention, boricacid should be loaded into the carrier so that it occupies up to 10% byvolume in a dried film, more preferably 0.3-5.0% by volume in a driedfilm, and most preferably 0.5-2% by volume in a dried film. Likewise,when used as an inhibitor, calcium dihydrogen phosphate can be added inan amount up to 10% by volume in a dried film, more preferably 0.3-7% byvolume, and most preferably 0.5-5% by volume in a dried film. Whencalcium, strontium phosphosilicate or calcium, strontium, zincphosphosilicate is used as an inhibitor, it can be added in an amount upto 25% by volume in a dried film, more preferably 0.3-10% by volume, andmost preferably 1-5% by volume in a dried film. If(2-benzothiazolylthio)succinic acid or the fatty amine salt of(2-benzothiazolylthio)succinic acid is used as an inhibitor in thecoating composition of the present invention, it can be loaded into thecarrier in an amount up to 10% by volume in a dried film, morepreferably 0.3-5% by volume, and most preferably 0.5-3% by volume in adried film. Sodium titanium oxide can be added in an amount up to 10% byvolume in a dried film, more preferably 1-10% by volume, and mostpreferably 3-6% by volume in a dried film. Zinc phosphate or zinccyanamide, when used in the present invention can be added in an amountup to 15% by volume in a dried film, more preferably 1-10% by volume ina dried film, and most preferably 3-6% by volume.

The carrier of the present invention is any compound which is capable ofadhesion to a metal surface, and is also capable of placing thecombination of inhibitors in proximity with the metal surface. Suitablecarriers include both curable and noncuring sealants, as well as curableprimer and unicoat systems. The combination of inhibitors of the presentinvention can also be incorporated into water-containing orwater-absorbing fluids that might cause corrosion when used in theproximity of metal such as de-icing liquids and coolants which areprimarily comprised of glycol and water. Other carriers include, but arenot limited to, aqueous solutions, amine-cured epoxy coatings, polyesterand urethane coatings, sealant matrices such as those based onepoxy-cured polythioether polymers, MnO₂ cured polysulfide sealants,non-curing end-capped polysulfide sealants, and other carriers as areknown in the art.

Additional materials can be added to the coating composition of thepresent invention such as pigments, rheological agents, adhesionpromoters and other additives, as are known in the art. For example, anoily, hydrophobic additive can be used with some of the inhibitorcombinations of the present invention to achieve more acceptable barrierperformance of sealants in water. Additionally, since the inhibitoringredients of this invention do not necessarily enhance adhesion as dooxidizing inhibitors such as chromate, a porous conversion coating oranodized layer on the substrate is required in very wet environments forgood mechanical adhesion of a coating or sealant to the metal.

As has been discussed above, chromate has been identified as anexcellent inhibitor because it performs the following four necessaryfunctions:

1. Rapid polymer exit and short term metal passivation.

2. Passivation by metal surface adsorption and double layer space chargealtering.

3. Forming a water insoluble passivation layer which persists or remainsinsoluble in neutral, alkaline and acidic environments.

4. Passivation by control of pH or neutralization of acid at themetal/electrolyte interface.

Therefore, in order to replace chromate, any combination of inhibitorsmust perform at least some, if not all of these functions in order tooperate as a successful corrosion inhibitor. Additionally, transport ofitself and other inhibitors out of a solid, e.g. polymeric, carriermatrix onto adjacent exposed bare metal areas in a moist but not liquidimmersed environment is a performance parameter exhibited by one of theinhibitors (boric acid) of this invention that is not exhibited bychromate.

In general, the first and second inhibitive functions are performed byone or more moderately water-soluble, fast but reversibly adsorbingingredients which exit readily from a coating or sealant, to give rapid,short-term passivation of a metal surface. Boric acid appears to havethe predominant fast or first-response effect out of a polymer matrix,and appears to also aid in releasing other, less soluble, more permanentpassivators from the polymer.

The third inhibitive function is performed by a slower respondingcombination of two ingredients to form a water-insoluble,acid-resistant, "persisting" passivation layer. By "persisting" is meantremaining on a metal surface that was exposed to inhibitor-containingsolution after that inhibitor-containing solution is removed andreplaced with a corrosive solution such as aqueous sodium chloridecontaining no inhibitor. This layer is believed to form on the baremetal as well as on the oxide. The combination of zinc phosphate or zinccyanamide and (2-benzothiazolylthio)succinic acid or its fatty aminesalt perform this function.

The fourth inhibitive function of these systems consists of bufferingthe pH, or controlling (neutralizing) the acidification of themetal-electrolyte interface environment in anaerobic or creviceconditions. Acid conditions dissolve the protective oxide and do notallow new insoluble oxide to form. In the formulations described by thepresent invention, dihydrogen phosphates, monohydrogen phosphates,pyrophosphate, orthosilicate, titanate, phosphosilicates and cyanamidecan perform this function.

A variety of different quantitative and qualitative test methods havebeen employed to identify and corroborate passivation behavior onaircraft aluminum. These include:

1. Galvanic Current Measurement (on solutions, coatings and sealants)

a. Uses a titanium cathode electrically shorted to the active metal,usually Al alloy, immersed in aqueous NaCl solution. Stainless steel, Cdplated steel and carbon composite cathodes are also used.

b. Current between anode and cathode (Al and Ti or other, respectively)is measured at regular intervals and plotted versus time.

c. Configuration accelerates development of acidic crevice conditions ina narrow, air-deficient gap between the parallel, spaced-off anode andcathode metals.

d. Used to quantitatively test functions 1, 2 and/or 4.

e. We define good performance as 2-5 times the initial (i.e., within 24hours) galvanic current reduction compared to uninhibited systems, withno rise over time out to 5-6 weeks. (Uninhibited coating and sealantinitial currents are approximately 0.5-1 microamps per square centimeteron bare 2024 alloy with a Ti cathode, with a visual rise in current of2-5 times within 1-2 weeks. Chromate gives approximately 0.2-0.3microamps per square centimeter initially, with no rise over time. Thecorresponding values are approximately 3-5 times higher for bare 7075alloy with a Ti cathode.)

2. Electrochemical Impedance Spectroscopy (EIS) (on solutions, coatingsand sealants)

a. Uses an active metal (e.g. Al alloy) working electrode, passivatedstainless steel counterelectrode, calomel reference electrode andSchiumberger potentiostat and frequency response analyzer.

b. Run in one or more of the following configurations:

i. Neutral or pH-adjusted, aerated NaCl solution, containinginhibitor(s).

ii. Active working electrode covered with inhibitor-containing, bondedcoating or sealant film exposed to open, aerated NaCl solution on theupper film side. Measures barrier properties orcorrosive-environment-penetration resistance (R_(pore)) of bonded films.

iii. Active working electrode covered with inhibitor-containing,non-bonded, "free film" of coating or sealant, exposed to open, aeratedaqueous NaCl solution on the upper side. In this configuration, when thefilm is penetrated with liquid, the whole air-deficientelectrolyte/active metal interface area is wetted and therefore knowndimensionally to calculate accurate charge transfer resistances(R_(ct)).

c. Used quantitatively to test functions 2,3 and/or 4.

d. We define good electrochemical performance as R_(ct) 's of greaterthan 10⁶ ohm-square centimeters, persisting over time. We define goodbarrier performance as exhibiting high (greater than 10⁸ ohm-squarecentimeters), essentially immeasurable R_(pore) 's for as long aspossible. We judge by making comparisons at equal film thicknesses. LowR_(pore) can be compensated for to a great extent by high R_(ct) as isthe case with chromate.

3. Filiform Testing (on coatings only)

a. Uses brief (usually one hour) HCl vapor exposure to initiate acidicfiliform (underfilm crevice) corrosion conditions at cut edges ofcoatings. Subsequent 80+% humidity exposure propagates further filiformcorrosion.

b. Relative effectiveness of inhibitors are compared on the samesubstrates. Differences in corrosiveness of Al substrates (variousalloys, Alclad, conversion-coated) usually give more variation for agiven coating than the differences between inhibitors, including none,in the same coating, on the same substrate.!

c. Use to qualitatively test function 4 and combined effects offunctions 1-3.

d. Good performance for a coating or coated system is usually defined asthe development from a cut edge of filiforms no greater than 3-6millimeters in length, with the majority less than 3 millimeters, after1000 or more hours.

4. Salt Spray Testing (on coatings and sealants)

a. Uses condensing corrosive (5%) salt solution to cause electrochemicalactivity of soluble inhibitors to be a factor, along with barrierproperties, but the effects cannot be separately determined.

b. By leaving some uncoated area exposed, ability of inhibitors to movefrom the resin matrix to uncovered or damaged areas and to protect themis determined.

c. As in filiform testing, relative effectiveness of inhibitors arecompared on the same substrates.

d. Used to qualitatively test function 1 and combined effects offunctions 2-4.

e. Good performance is usually defined as no corrosion blister formationaway from cut edges and minimal undercutting or corrosion blisterformation at cut edges. Judgments are usually based on visualcomparisons over time with uninhibited and chromate-inhibited controls.

5. pH Range Immersion Testing (on solutions only--solutions containinginhibitors, buffered from pH 3-10)

a. Found to be a predictor of "persisting" (insoluble) passivation layerformation and of resistance to prolonged-exposure, acidic-conditioncrevice corrosion.

b. Used to test function 3.

c. We define good performance as resistance to visual corrosion on flatsurfaces and cut edges of bare metal over a wide pH range for as long aspossible; on more than one alloy, if possible. Judgments are based onvisual comparisons over time with uninhibited and chromate inhibitedcontrols.

In order to work at all, effective inhibitors must exhibit at leastfunction 2, corrosion current suppression. Functions 1, 3 and 4 increasetheir effectiveness. In addition, whether they perform on one or both ofthe most common aerospace aluminum substrates (AA2024-T3 or AA7075-T6)(function 5), and whether their addition significantly lowers the waterresistance/adhesion of a polymer matrix (function 6) is considered.

The following table summarizes some of the preferred inhibitorcombinations, rated according to how many of the above six functionsthey perform. This invention, however, is not limited to thecombinations below. These abbreviations have been used to identify thevarious inhibitors in which:

B=boric acid

C=calcium dihydrogen phosphate

K=dipotassium monohydrogen phosphate

A=ammonium dihydrogen phosphate

P=sodium pyrophosphate

H=calcium, strontium phosphosilicate or calcium, strontium, zincphosphosilicate

S=tetrasodium orthosilicate

I=(2-benzothiazolylthio)succinic acid or the fatty amine salt of(2-benzothiazolylthio)succinic acid

N=sodium titanium oxide

Z=zinc phosphate

W=zinc cyanamide

    ______________________________________    PREFERRED INHIBITOR COMBINATIONS    GOOD (1-2 functions)                BETTER (3-4 functions)                               BEST (5 functions)    Inhibitor           Function Inhibitor                             Function                                     Inhibitor                                            Function    ______________________________________    CN     2,5      BINZ     1,2,3,5 BCINZ  1,2,3,4,5    KI     2,5      BCNZ     1,2,4,5 BCINW  1,2,3,4,5    PS     2,4      BNZ      1,2,5   HINZ   2,3,4,5,6                    IZ       2,3,6   CINZ   2,3,4,5,6                    INZ      2,3,6                    HN       2,5,6   SrCrO.sub.4                                            2,3,4,5,6                    HINS     2,4,5    ______________________________________     Functions     1 = exposed metal passivation adjacent to polymer     2 = low galvanic current, high EIS Rct     3 = formation of "persisting layer     4 = crevice condition low galvanic current     5 = performs on both 2024 and 7075 alloys     6 = good water swell resistance (low water solubility)

The present invention will best be illustrated by the following tablesof examples. The same inhibitor abbreviations set forth above have alsobeen used in these tables. A series of compositions were prepared andtested on several light metal alloys. The test samples were preparedaccording to the tables set forth below, all components being mixedtogether in a vessel, and the various formulations were applied to 2024and 7075 alloy substrates. The samples were run through a series oftests to test their corrosion-inhibitive properties. These tests havealready been discussed, with the results summarized above. In theexamples, as in the rest of the specification and claims, allpercentages are by volume in a dried film.

    ______________________________________    FORMULATIONS (as volume % of non-volatiles)           Non-curing            Solvent-borne Primer    Com-   Sealants   Cured Sealants                                 Coatings    ponents           HINZ   BCINZ   BCINZ CINZ BCINZ BCNZ  BCINW    ______________________________________    Non-   71.1   69.9    --    --   --    --    --    curing    polysul-    fide resin    Epoxy- --     --      73.7  73.6 --    --    --    cured    polythio-    ether    resin    Amine-    cured  --     --      --    --   64.0  64.0  75.2    epoxy    resin    Adhesion           1.0    1.0     0.2   0.2  4.0   4.0   10.1    promoters,    disper-    sants,    flow    agents    Inert  21.8   21.3    9.0   9.0  13.4  14.8  2.3    fillers    H      1.2    --      --    --   --    --    --    B      --     2.2     2.2   --   2.9   2.9   1.4    C      --     1.4     2.0   4.0  2.9   2.9   1.4    I      0.6    0.6     6.5   6.6  1.4   --    0.7    N      1.2    1.1     2.0   2.1  5.7   5.7   4.8    Z      3.1    2.5     4.4   4.5  5.7   5.7   --    W      --     --      --    --   --    --    4.1    Solvent           (+20)  (+20)   (+10) (+10)                                     (+70) (+70) (+320)    Water  --     --      --    --   --    --    --    ______________________________________    Water-borne Primer Coatings    Components            BCINZ   BCINW   BCNZW  BCNZ  BCNW  CINZ    ______________________________________    Non-curing            --      --      --     --    --    --    polysulfide    resin    Epoxy-cured            --      --      --     --    --    --    polythioether    resin    Amine-cured            69.1    69.1    69.1   69.1  69.1  69.1    epoxy resin    Adhesion            1.7     1.7     1.7    1.7   1.7   1.7    promoters,    dispersants,    flow agents    Inert fillers            9.9     9.9     11.5   14.3  15.9  13.2    H       --      --      --     --    --    --    B       3.3     3.3     3.3    3.3   3.3   --    C       3.3     3.3     3.3    3.3   1.7   3.3    I       1.7     1.7     --     --    --    1.7    N       5.5     5.5     5.5    5.5   2.8   5.5    Z       5.5     --      2.8    2.8   --    5.5    W       --      5.5     2.8    --    5.5   --    Solvent (+28)   (+28)   (+28)  (+28) (+28) (+28)    Water   (+200)  (+200)  (+200) (+200)                                         (+200)                                               (+200)    ______________________________________

It will be appreciated from the foregoing, that a non-chromatecontaining coating composition having excellent properties forinhibiting corrosion of a variety of metal surfaces may be prepared fromthe synergistic combination of two to six individual corrosioninhibitors contributing two to five separate functions to the inhibitionof metals, such as light metal alloys. The specific components of thecomposition will depend upon particular applications and factors such asthe metal alloy substrate, the particular polymer or solution matrix inwhich the inhibitors are carried, and the range of exposure conditionsthe material will see in its particular location (e.g. aircraftinterior, exterior, fuel tank, skin coating, lap seam, etc.).

The foregoing discussion and examples are merely meant to illustrateparticular embodiments of the invention, and are not meant to belimitations upon the practice thereof. It is the following claims,including all equivalents, which define the scope of the invention.

We claim:
 1. A non-chromate, corrosion-inhibiting coating compositionfor metal surfaces, said composition containing inhibitors, saidinhibitors comprising:a synergistic amount of at least one compoundselected from the group consisting of boric acid; borate dipotassiummonohydrogen phosphate; calcium dihydrogen phosphate; ammoniumdihydrogen phosphate; sodium dihydrogen phosphate; potassium dihydrogenphosphate; sodium pyrophosphate; calcium, strontium phosphosilicate;calcium, strontium, zinc phosphosilicate; tetrasodium orthosilicate; andmixtures thereof; a synergistic amount of sodium titanium oxide; asynergistic amount of at least one compound selected from the groupconsisting of zinc phosphate, zinc cyanamide, and mixtures thereof; andan effective film-forming amount of a carrier for said inhibitors, saidcarrier capable of placing said inhibitors in proximity with said metalsurface.
 2. The coating composition of claim 1 and further comprising asynergistic amount of at least one compound selected from the groupconsisting of (2-benzothiazolylthio)succinic acid, fatty amine salt of(2-benzothiazolylthio)succinic acid, and mixtures thereof.
 3. Thecoating composition of claim 1 wherein said inhibitors comprise 3-40percent by volume in a dried film.
 4. A non-chromate,corrosion-inhibiting coating composition for metal surfaces, saidcomposition containing inhibitors, said inhibitors comprising:asynergistic amount of at least one compound selected from the groupconsisting of boric acid, borate, calcium dihydrogen phosphate, andmixtures thereof; or a synergistic amount of at least one compoundselected from the group consisting of calcium, strontiumphosphosilicate; calcium, strontium, zinc phosphosilicate; and mixturesthereof; a synergistic amount of at least one compound selected from thegroup consisting of (2-benzothiazolylthio) succinic acid, the fattyamine salt of (2-benzothiazolylthio) succinic acid, and mixturesthereof; a synergistic amount of sodium titanium oxide; a synergisticamount of at least one compound selected from the group consisting ofzinc phosphate, zinc cyanamide, and mixtures thereof; and an effectivefilm-forming amount of a carrier for said inhibitors, said carriercapable of placing said inhibitors in proximity with said metal surface.5. The coating composition of claim 4 wherein said boric acid comprises0.3-10 percent by volume in a dried film.
 6. The coating composition ofclaim 4 wherein said calcium dihydrogen phosphate comprises 0.3-10percent by volume in a dried film.
 7. The coating composition of claim 4wherein said phosphosilicate comprises 0.3-25 percent by volume in adried film.
 8. The coating composition of claim 4 wherein said succinatecompound comprises 0.3-10 percent by volume in a dried film.
 9. Thecoating composition of claim 4 wherein said sodium titanium oxidecomprises 1-10 percent by volume in a dried film.
 10. The coatingcomposition of claim 4 wherein said zinc phosphate and zinc cyanamidecomprise 1-15 percent by volume in a dried film.